A light emitting device refers to a semiconductor device capable of implementing light of various colors by configuring a light emitting source with various compound semiconductor materials such as GaAs, AlGaAs, GaN, InGaAlP, and the like. Light emitting devices, advantageously having an excellent monochromatic peak wavelength and excellent optical efficiency, being compact and environmentally friendly, and consuming low levels of power, and the like, have been widely used for various applications such as in TVs, computers, illumination devices, vehicles, and the like, and the utilization thereof is gradually expanding.
Recently, organic light emitting diodes (OLEDs) using organic compounds, rather than inorganic compounds, have been increasingly applied to products. OLEDs, able to be implemented in a large area and easily bendable, are anticipated to be extendedly used in various fields of application.
A light emitting device (such as an LED) has characteristics that a current flowing therethrough is increased exponentially in a voltage (e.g., over a voltage applied to both ends thereof). Thus, in a case in which a lighting device using LEDs as light sources is driven upon receiving direct current (DC) power voltage with fluctuations therein, a constant current circuit generating a constant current or a DC/DC converter maintaining a constant output voltage is generally used. Namely, in an LED, a current is very susceptible to change, with regard to an applied voltage, and thus, in order to apply DC power with large fluctuations therein to an LED and obtain stable optical output, an apparatus or a method for stably controlling a current flowing in an LED is required.
FIG. 1 schematically illustrates a related art LED driving circuit to which alternating current (AC) power is applicable, and voltage and current waveforms of the LED driving circuit. Specifically, FIG. 1A is a view schematically illustrating a related art LED driving circuit, FIG. 1B is a view illustrating a waveform of a voltage VDR applied to a light source unit D and a resistor R in FIG. 1A, and FIG. 1C is a view illustrating a waveform of a current ID flowing in the light source unit D. First, referring to FIG. 1A, the related art LED driving circuit includes a rectifying unit converting alternating current (AC) power input from the outside into DC power, the light source unit D driven upon receiving a DC voltage output from the rectifying unit and including a plurality of LEDs, and a resistor R connected to the light source unit D in series.
As mentioned above, with respect to an input voltage, a current flowing in an LED is changed exponentially, so the resistor R may be connected to the light source unit D including a plurality of LEDs in series to restrain a change in the current flowing in the light source unit D, and a peak current flowing in the LED may be prevented from being changed exponentially according to fluctuations (e.g., 220 Vrms→240 Vrms) in the AC power voltage input from the outside due to the resistor R. Here, if a value of the resistor R may be increased, a variation of a peak current flowing in the LED may be reduced but a proportion of power consumed in the resistor R is increased, and a peak current flowing in the LED when a voltage is the highest has a very high value, relative to an average or root mean square (RMS) current, increasing a peak factor (or crest factor). Also, as illustrated in FIG. 1C, since a current flows only in a partial section of the overall period, it may have difficulty in satisfying the International Electrotechnical Commission (IEC) regarding electricity usage, such as power factor as an indicator indicating similarity between an input voltage and a current waveform, a magnitude of a harmonic component (harmonic distortion) included in an input current, and the like. Also, a current flowing in the LED is changed relatively significantly according to a variation of an AC power voltage input from the outside, making it difficult for the LED driving circuit to be applied to a case in which fluctuations in an input power voltage are large.
FIG. 2 is a view illustrating a modification of a related art LED driving circuit that may be applicable to commercial AC power and voltage and current waveforms of the LED driving circuit. Referring to FIG. 2A, the related art LED driving circuit includes a rectifying unit converting AC power input from the outside into DC power, a light source unit D including a plurality of LEDs and driven upon receiving DC power output from the rectifying unit, and a current limiting unit IS connected to the light source unit D in series to limit a current input to the light source unit D. The current limiting unit IS operates as a current source only when a forward voltage has a magnitude equal to or greater than a predetermined value in a direction in which a current flows. FIG. 2B illustrates a waveform of a voltage VDR applied to the light source unit D and the current limiting unit IS, and FIG. 2C illustrates a waveform of a current ID flowing in the light source unit D and the current limiting unit IS. In the case of using the current limiting unit IS, the same average value of the current flowing in the light source unit D as that in case of using the resistor R (please see FIG. 1), while lowering a peak value of the current flowing in the light source unit D, can be obtained.
In the LED driving circuit illustrated in FIG. 2, even in the case that a voltage of external AC power is increased (e.g., 220 Vrms→240 Vrms), the current ID flowing in the light source unit D is rarely affected. In this case, however, since a current-voltage relationship of the LED appears exponentially, if a voltage across the light source unit D is lower than a predetermined voltage, the current is rapidly reduced and rarely flows. Thus, even in the LED driving circuit illustrated in FIG. 2, in the section P in which the input voltage is lower than a rated voltage of the LED, a current flows rarely, and thus, a waveform of the current ID of the light source unit D is significantly different from the rectified sinusoidal wave and a peak value of the current ID is still high, relative to the rectified sinusoidal wave having the same RMS value.